Regardless of the kinds of vehicles, tires of the vehicles must be inflated to a proper pressure. When tire pressure is too low or tire temperature is too high, driving safety is adversely affected. In addition, due to high rotational speeds when driving and imperfect airtightness of the tire valve stem, air inside the tire gradually leaks out, which causes the tire pressure to be too low and thus decreases driving safety. Therefore, many tire pressure indication devices have been developed for displaying tire pressure so a user can know when the tire pressure is abnormal.
Taiwan Utility Model Patent No. M261387 discloses a tire pressure alarm device which includes a metal casing enclosing a lateral separating seat and a protruding shoulder formed near the top thereof; a valve nozzle disposed at the bottom of the metal casing; a membrane disposed at the top surface of the lateral separating seat; a sleeve received in an upper chamber and locating on the membrane with a through-hole defined in the top portion thereof; a pushing rod received in the sleeve with the bottom portion thereof abutting against the membrane, and with the top portion thereof extending through the through-hole of the sleeve, and a laterally protruding ring formed near the bottom thereof; a resilient member disposed between the protruding ring of the pushing rod and the top of the sleeve and covering the pushing rod; a metal contact board disposed at the top portion of the pushing rod and disposed astride the protruding shoulder of the metal casing, wherein when the tire pressure is normal, the metal contact board is upwardly pushed by the tire pressure and departs from the protruding shoulder, and when the tire pressure is insufficient, the metal contact board moves downwardly and contacts the protruding shoulder; a battery device disposed at the top surface of the contact board and contacting the metal contact board with a negative pole thereof; a conductive member connected with a positive pole of the battery device; a transmission device disposed at the top portion of the conductive member and of the metal casing; and a cover disposed at the periphery of the metal casing. The tire pressure alarm device employs the resilience of the resilient member to confront the tire pressure, that is, the resilience of the resilient member is employed as the desired value of the tire pressure. When the tire pressure is insufficient, the tire pressure fails in confronting the resilience of the resilient member, and so fails in upwardly pushing the resilient member and the pushing rod and the metal contact board on the resilient member. Thus, the metal contact board keeps contact with the protruding shoulder of the metal casing and so switches on the power supply circuit of the battery device. Therefore, the transmission device transmits a signal which is received by a receiver disposed near the driver's seat for alarming the driver.
The tire pressure alarm device employs the resilience of the resilient member as a desired value of the tire pressure and controls switching on or off of the circuit through pushing a related mechanical module (the membrane, the pushing rod and the metal contact board) being pushed or not. The mechanical module can only detect at a lower pressure point. Furthermore, the tire pressure alarm device transmits an alarm signal from the transmission device only when the tire pressure is insufficient. However, the tire pressure alarm device cannot display the tire pressure status in real time in operation. When the transmission device of the tire pressure alarm device does not transmit an alarm signal, it does not mean that the tire pressure is normal because it is possible that the battery has no power or the mechanical module is damaged. Therefore, the tire pressure and temperature value cannot be provided in real time, which results in inconvenience of determining the status of the tire and adversely affects the safety of driving.